In a gas chromatograph, a sample is introduced onto the head of a gas chromatograph column and is carried towards the detector by a carrier gas. The sample components become separated from one another as they travel the length of the column by virtue of differences in their rates of interaction with the sorptive material coating the inside walls of the column or coated onto a substantially inert support material packed into the column. The different sample components are therefore retained for different lengths of time within the column, and arrive at the detector at characteristic times. These "retention times" are used to identify the particular sample components, and are a function of the type and amount of sorptive material in the column, the column length and diameter, the carrier gas type and flowrate, and of the column temperature. In order to have repeatable retention times, the column temperature must also be repeatable. Because a gas chromatograph must operate in a range of ambient temperatures, the gas chromatograph must be controllably heated to a temperature greater than the highest expected ambient temperature in order to achieve repeatable retention times.
The gas chromatograph analysis is complete once all the sample components have traversed the column length and have flowed through the detector. The analysis time is therefore determined by the retention time of the most-retained sample component.
If a second sample is introduced onto a gas chromatograph column before the first analysis is completed, the potential exists for little-retained components from the second sample to flow through the detector concurrently with more-retained components from the first sample. This situation leads to difficulty in interpreting the detector output signal. To avoid this situation a technique known as "precolumn backflushing" is used. Two gas chromatograph columns, typically a shorter precolumn and a longer analytical column, are joined in series with a tee between the columns. Initially, the branch of the tee is closed to flow and carrier gas is directed along the precolumn and analytical column in series. A sample is introduced onto the head of the precolumn and carried along the precolumn towards the analytical column. After an interval, and using suitable carrier gas switching valves, vents and control timing, the carrier gas is introduced at the branch of the tee between the two columns. Sample components which have already passed the tee continue to be carried towards the detector along the analytical column. Those which are still in the precolumn are flushed backwards along the precolumn and vented from the head of the precolumn. Backflushing of the precolumn to clean it for the next sample occurs concurrently with analysis on the analytical column. Once all components have eluted from both columns, the carrier gas is switched to flow through both columns in series and the system is ready to accept the next sample.
Precolumn backflushing beneficially reduces total analysis time because it is not necessary to wait for the more retained components which are not required to be analyzed to traverse the analytical column. Hence, the analytical confidence is increased by reducing the possibility of coelution.
In a gas chromatograph, the types of sample components which can be separated from one another prior to detection are determined by the type of gas chromatograph column used. Some column types are suited to light gases, others to heavier vapors, and yet others to polar vapors. For flexibility in types of chemical components which can be analyzed in a gas chromatograph, it is beneficial to have several different types of columns available for installation so that the optimum column can be selected for the particular analysis desired.
When a large volume of a sample exists, as in ambient air sampling, a sample can be introduced automatically into a gas chromatograph by means of a gas sampling loop. A suction pump draws the sample through a defined volume. With appropriate valving and controls, the carrier gas is momentarily diverted through this volume to deliver the entire sample, or a defined aliquot of it, onto the head of the column for analysis. Where only a small volume of sample exists, as in a vessel of a sample collected from a process or from a remote location, an aliquot of the sample can be introduced manually onto the head of the gas chromatograph column using a hypodermic type syringe. An appropriate syringe injection port with resealable septum is fitted at the column head. For flexibility in analyzing samples from both types of sources, it is beneficial for a gas chromatograph to provide for both types of sample introduction.
In a gas chromatograph it is beneficial for all parts which contact the sample to be chemically-inert, non-sorptive and also to be maintained at an elevated temperature. The quality of the analysis is preserved by reducing sample degradation through chemical reaction of sample components with and adsorption of sample components onto gas chromatograph parts. Minimizing the sample flow path further reduces the possibility of sample degradation by contact.
In a hand-carried, battery-powered portable gas chromatograph it is desirable to have a low mass for ease of portability, and also to have a long operating time so that the user need not carry extra batteries or return frequently to a battery-charging station. Lightweight and low-power electronic components are available for instrumentation amplifiers, displays, indicators, and controls so that most of the power consumed by a gas chromatograph is used in elevating the temperature of the gas chromatograph's parts and in maintaining the parts at elevated temperature.